In some heavy water-moderated nuclear reactors, for example Candu reactors, fuel for the nuclear reaction is carried in an array of pressure tubes disposed within a calandria vessel forming the reactor core. Heavy water is circulated through the pressure tubes and is heated by the nuclear reaction; the heated heavy water can then be used to drive turbines for generating electricity. Each pressure tube is surrounded by and coaxial with a calandria tube and the nuclear reaction is moderated by heavy water disposed within the calandria vessel externally of the calandria tubes. The temperature of this heavy water is significantly lower than the temperature of the heavy water in the pressure tubes when the reactor is in operation. Each pressure tube is spaced from the associated calandria tube by means of spacers called garter springs which are fitted around the pressure tubes at intervals along its length. However, it has been found that, over a period of time, the garter springs tend to migrate along the pressure tubes under the influence of vibrations generated in normal operation of the reactor and that the garter springs may eventually move to positions at which they no longer act as effective spacers, permitting the pressure tubes to sag.
It is therefore desirable to be able to reposition the garter springs to ensure proper spacing between the pressure tubes and the calandria tubes. The difficulty is that the garter springs are not directly accessible since the space between the two tubes is closed in the assembled reactor. Disassembly of the pressure tubes and calandria tubes is not practicable because the tubes are radioactive and such disassembly would be very expensive.
Accordingly, a specific object of the invention is to provide a non-intrusive, non-destructive method for repositioning garter springs or other conductive bodies which are shielded by a relatively conductive wall (e.g. the wall of the pressure tube).